Effects of oscillating dietary crude protein concentrations on production, nutrient digestion, plasma metabolites, and body composition in lactating dairy cows

Abstract

We hypothesized that dairy cows fed oscillating metabolizable protein (MP) and crude protein (CP) concentrations on a 24-h frequency for a diet formulated to be below MP requirements would use N more efficiently (i.e., increased milk protein yields and less manure N) without increasing mobilization of body protein stores than would cows fed the same deficient MP diet continuously, although both treatments would on average have equal MP concentrations. In a randomized block design, 30 Holstein cows (119 ± 21 d in milk; 667 ± 69 kg of body weight) were blocked according to milk yield within a parity (3 primiparous and 7 multiparous blocks) and fed 1 of 3 treatments: (1) diet with 16.2% CP (109% of MP requirements) fed continuously (109MP), (2) diet with 14.1% CP (95% of MP requirements) fed continuously (95MP), or (3) diets oscillating on a 24-h cycle from the 109MP diet and a diet with 11.9% CP (~78% of MP requirements) such that average CP and MP concentration would be the same as 95MP (OSC). Dry matter intake was similar between 109MP and 95MP (22.9 vs. 23.2 kg/d) but tended to be lower for OSC (22.2 kg/d) compared with 95MP. Milk yield was greater for 109MP compared with 95MP (36.6 vs. 35.1 kg/d) and similar between 95MP and OSC (35.3 kg/d). Milk protein and energy-corrected milk yields were similar among treatments. Milk urea N (MUN) concentration was higher for 109MP compared with 95MP (12.8 vs. 10.2 mg/dL), and tended to be higher for OSC (10.9 mg/dL) compared with 95MP. Higher MUN concentration for OSC occurred despite lower N intake (474 vs. 512 g of N/d) and similar milk N outputs compared with 95MP (164 vs. 179 g/d). On days when cows on OSC were fed high versus low MP diets, yields of milk (34.8 vs. 36.3 kg/d) and milk protein (1.0 vs. 1.1 kg/d) and MUN concentration (9.3 vs. 12.5 mg/dL) followed the oscillation pattern but lagged the change in diet CP by 1 d, whereas dry matter intake, yields of milk fat, plasma energy metabolites, AA, and 3-methyl-His were similar between days. Nutrient digestibility was similar for major nutrients across treatments except for CP, which was greater for 109MP (65.2%) and OSC (65.3%) compared with 95MP (61.7%). Compared with 95MP, OSC did not increase milk N relative to N intake (averaged 0.35 g of milk N/g of N intake) or N balance; however, urinary N output was increased for OSC versus 95MP (0.32 vs. 0.24 g of urine N/g of N intake). Body composition estimated using urea dilution was similar across treatments, and all cows accreted lipid and energy during the trial. Empty body CP did not change over the 50-d treatment period. Overall, greater CP digestion, urinary N excretion, and MUN concentrations with lesser N intake and similar milk N outputs for OSC compared with 95MP suggests that the lower energy intake by OSC cows may have limited potential responses to altered N metabolism.

Dietary protein oscillation: Effects on feed intake, lactation performance, and milk nitrogen efficiency in lactating dairy cows

“…Effects of dietary CP oscillation in ruminants remain ambiguous, and reported effects on NUE range from considerably negative (Ludden et al, 2002a) to substantially positive (Cole, 1999;Archibeque et al, 2007b;Doranalli et al, 2011). Recently, Tebbe and Weiss (2020) demonstrated that dairy cows receiving an oscillating CP diet (11.9% and 16.2% CP, 24-h basis) performed similarly in NUE, milk protein, and ECM yield to cows receiving a static CP diet (14.1% CP), in agreement with the present experiment. However, their reported lower N intake by cows with oscillating diet contrasts with our lack of treatment effect on N intake.…”

“…Time frequency of the oscillation was different between these experiments, but amplitude and concentration of dietary protein, phase of lactation, and parity were comparable between studies. Reduced N intakes may therefore be related to a shorter frequency of oscillation used by Tebbe and Weiss (2020). In line with this, in our experiment, DMI of OS cows tended to be smaller than that of ST cows in the first 24 h, but not the subsequent 24 h, within a 48-h phase of feeding LP.…”

“…A trend for an increase in N digestibility, greater MUN, and urinary N excretion (as % of N intake) in oscillating cows occurred, despite a lower N intake (Tebbe and Weiss, 2020). Based on improvements in production or efficiency with growing ruminants using 48-h periods of oscillations (Cole, 1999;Archibeque et al, 2007b;Doranalli et al, 2011), MRT measured in lactating dairy cows (marginally lower than 48 h, Warner et al, 2013), and lack of improvement in NUE when CP concentration was oscillated on a 24-h basis (Tebbe and Weiss, 2020), we hypothesized that a 48-h period of oscillation may be a suitable dietary strategy to improve NUE in lactating dairy cows. Our objective was to determine if an oscillating diet (OS; 48-h phases of 13.4 and 16.5% CP, DM basis, respectively) compared with a static diet (ST; 14.9% CP) would increase NUE in lactating dairy cows.…”

“…This would allow for a concurrent relative excess of postruminal N absorption and deficiency in ruminal N during days of feeding the low-CP diet, resulting in a simultaneous increase and reduction in concentrations of plasma urea and ruminal ammonia, respectively, conditions known to stimulate urea flux into the rumen (Kennedy and Milligan, 1980;Abdoun et al, 2006). Tebbe and Weiss (2020) demonstrated that dairy cows receiving a diet that oscillated between 11.9 and 16.2% CP on a 24-h basis performed similarly in nitrogen use efficiency (NUE; milk N/N intake) and yields of milk, milk protein, and ECM to cows receiving a static diet (14.1% CP). A trend for an increase in N digestibility, greater MUN, and urinary N excretion (as % of N intake) in oscillating cows occurred, despite a lower N intake (Tebbe and Weiss, 2020).…”

“…Tebbe and Weiss (2020) demonstrated that dairy cows receiving a diet that oscillated between 11.9 and 16.2% CP on a 24-h basis performed similarly in nitrogen use efficiency (NUE; milk N/N intake) and yields of milk, milk protein, and ECM to cows receiving a static diet (14.1% CP). A trend for an increase in N digestibility, greater MUN, and urinary N excretion (as % of N intake) in oscillating cows occurred, despite a lower N intake (Tebbe and Weiss, 2020). Based on improvements in production or efficiency with growing ruminants using 48-h periods of oscillations (Cole, 1999;Archibeque et al, 2007b;Doranalli et al, 2011), MRT measured in lactating dairy cows (marginally lower than 48 h, Warner et al, 2013), and lack of improvement in NUE when CP concentration was oscillated on a 24-h basis (Tebbe and Weiss, 2020), we hypothesized that a 48-h period of oscillation may be a suitable dietary strategy to improve NUE in lactating dairy cows.…”